The transaction ID is a merchant generated reference to a transaction. Typically this is also the same as the order ID from your order management system or just an incremental unique identifier for each request / transaction sent to Peach Payments for processing.
The transaction ID is supplied by the merchant in the API
request in the parameter IDENTIFICATION.TRANSACTIONID.
It is not generated by the Peach Payments' system but in fact is an essential part of the transaction request.